Thermistors are thermally sensitive resistors which have a large temperature coefficient of resistance. They are of two kinds. The first kind exhibits a positive change in resistance with increasing temperature (PTC) and the second kind exhibits a negative change in resistance with increasing temperature (NTC). NTC thermistors ordinarily consist of sintered semiconductive materials and can be used to make elements having resistance values of 10 to 1,000,000 ohms at room temperature. The operational range of such thermistors extends from 75 to 1275K. Therefore, they find extensive use as temperature sensors.
Thermistors are, however, used extensively for such other applications as an electronic time delay, capacitor, inductor in low frequency oscillators, surge suppressor, voltage or current limiter, gas pressure sensor, thermoconductivity detector, liquid or gas flow sensor and solid or liquid level indicator.
In whatever application they are used, it is essential that the thermistor have the following properties: (1) large temperature coefficient of resistance (TCR); (2) large range of available resistance values; (3) capability of operation with solid, liquid or gaseous environment over a reasonable temperature range; (4) adaptable to variety of sizes and shapes; and (5) ability to withstand mechanical and electrical stresses.